This section provides background information related to the present disclosure which is not necessarily prior art.
Press-hardened steel (PHS) is also referred to as “hot-stamped steel” or “boron-steel” (e.g., 22 MnB5 alloy) is one of the strongest steels used for automotive body structural applications, typically having tensile strength properties on the order of about 1,400 megapascals (MPa) or higher. Such a steel alloy has low manganese levels and no aluminum and exhibits desirable properties, including high strength-to-weight ratios. Components formed from PHS have become prevalent in various industries and applications, including general manufacturing, construction equipment, automotive or other transportation industries, home or industrial structures, and the like. PHS components are often used for forming load-bearing components, like structural pillars and door beams, which typically require high strength materials. Thus, the finished state of these steels are designed to have high strength and enough ductility to resist external forces, for example, to resist intrusion of external objects into the passenger compartment without fracturing, so as to provide protection to the occupants.
The PHS steel blank is then austenitized in a furnace. Austenitization is typically conducted in the range of about 880° C. to 950° C. The steel blank may then be hot stamped by being pressed and quenched in dies. In hot stamping of PHS, forming and hardening are combined into a single operation, which may be one of two main types of processes: indirect and direct. Under the direct method, the PHS component is formed and pressed simultaneously between dies, which quenches the steel. The dies may be water-cooled, for example. Under the indirect method, the PHS component is cold formed to an intermediate partial shape before austenitization and the subsequent pressing and quenching steps are then conducted. The quenching of the PHS component hardens the component by transforming the microstructure from austenite to martensite. After the typical indirect or direct PHS processes (after hot forming and quenching), the PHS high-strength steel microstructure is predominantly (e.g., greater than 98%) martensite.
PHS components may require cathodic protection. The PHS component may be coated prior to applicable pre-cold forming (if the indirect process is used) or austenitization. Coating the PHS component provides a protective layer to the underlying steel component. Such coatings typically include an aluminum-silicon alloy and/or zinc. Zinc coatings offer cathodic protection; the coating acts as a sacrificial layer and corrodes instead of the steel component, even where the steel is exposed. However, liquid metal embrittlement (LME) may occur when a metallic system is exposed to a liquid metal, such as zinc, during forming at high temperature, resulting in potential cracking and a reduction of total elongation or diminished ductility of a material. LME may also result in decreased ultimate tensile strength. To avoid LME in conventional PHS processes, numerous additional processing steps are conducted.
Alternative high-strength steel alloy materials to PHS alloy may be used to form press-hardened steel components, such as select high-strength transformation induced plasticity (TRIP) steel like delta-TRIP steel and medium manganese content TRIP steel. However, alternative hot-formed press-hardened structures formed from such select TRIP steels often have microstructures with retained austenite and thus may not have comparable high-strength or hardness levels to comparative PHS structures having fully martensitic microstructures. Further, when such select alternative high-strength TRIP steel alloy materials are galvanized or galvannealed and then press-hardened, they likewise may suffer from LME. Thus, there is an ongoing need for streamlined processes of forming high-strength hot-formed press-hardened steel components having necessary hardness and strength levels, while providing galvanic protection substantially free of LME.